Single side polishing of a wafer is performed by using a polishing apparatus composed of a turn table having a polishing pad attached thereto, a polishing agent supply mechanism for supplying a polishing agent onto the polishing pad, a polishing head for holding a wafer, etc. These apparatus polish a wafer by holding the wafer with the polishing head, supplying the polishing agent onto the polishing pad from the polishing agent supply mechanism, and bringing the surface of the wafer into sliding contact with the polishing pad while rotating each of the turn table and the polishing head (see Patent Document 1, for example).
In polishing a wafer with a polishing apparatus described above, the wafer sometimes comes off from the polishing head. In some cases, the polished wafer is left on the turn table due to a mistake in picking up. In order to detect these, polishing apparatuses are provided with a wafer-detecting sensor(s).
The wafer-detecting sensor is provided with functions to stop the polishing apparatus and to raise an alarm to let an operator know an accident when detecting coming off of a wafer from the polishing head during polishing in order to prevent a detached wafer from being in contact with another polishing head and broken, and to prevent continuing the polishing with the wafer being broken.
The wafer-detecting sensors have been disposed at two places between the adjacent polishing heads 102 on the turn table 104 as shown in FIG. 9 and FIG. 10. As the wafer-detecting sensors 106, a general optical fiber type, which take 500 μs to output a stop signal from detecting a wafer, have been used.
As the turn table driving mechanism for rotating the turn table 104 used in the previous polishing apparatus 101, a warm speed reducer 111 and a constant torque motor 112 for inverter driving shown in FIG. 11 have been used. The rotation of the constant torque motor 112 for inverter driving is transmitted to the speed reducer 111 via a pulley and a timing belt.
When the wafer-detecting sensor cannot detect a wafer that has come off from the polishing head 102, the wafer can collide to another polishing head on the same turn table and be broken. In a method to perform polishing by way of plural turn tables, called an index system, fine pieces of the broken wafer adhered on a polishing head can bring secondary contamination of an ordinal polishing pad 103 since the polishing head moves to another turn table to continue polishing.
When a wafer is broken during polishing, fine pieces of the wafer left on the polishing pad 103 can generate a scratch on the polishing pad 103. Accordingly, the polishing pad 103 is subjected to cleaning or exchanging. The polishing head 102 is also exchanged since the polishing head 102 cannot be used due to the broken wafer left on the absorption surface of the polishing head 102 and a retainer guide. In a polishing system to use a polishing agent while recycling thereof, the polishing agent has to be cleaned in order to remove the fine pieces of a wafer penetrated in a circular line of the polishing agent. As described above, when a wafer is broken during polishing, the polishing is suspended for a long time to restore the polishing apparatus, which lowers the productivity.
When a wafer comes off from the polishing head 102, the wafer-detecting sensors 106 detects the wafer by light reflection, and suddenly stops the rotation of the motor 112 of the turn table driving mechanism. However, in an apparatus with the turn table driving mechanism having a power transmittance part by mechanical contact such as a speed reducer as described above, an immediate stop is actually impossible since it causes a damage of the apparatus such as a breakage of a gear, tooth skipping and wearing of a belt in the speed reducer. Moreover, the motor does not have a function which can forcibly stop itself, thereby continuing the rotation with an inertia in proportion to the weight of the turn table 104. In this case, the rotation of the turn table 104 has been stopped by pressure of the polishing heads 102 to press the turn table 104.
In the previous polishing apparatus described above, collision of a wafer can be avoided when the rotation rate of the turn table is low, with procedures of detecting coming off of the wafer from the polishing head during polishing with the wafer-detecting sensor and then stopping the turn table. When the rotation rate of the turn table is high or middle, however, the turn table cannot be stopped, thereby causing collision of a wafer if the wafer-detecting sensor detects coming off of the wafer from the polishing head during polishing and begins to stop the turn table.